BIOLOGICAL EVALUATION OF RATIONALLY MODIFIED ANALOGS OF THE H-TYPE-IIBLOOD-GROUP TRISACCHARIDE - A CORRELATION BETWEEN SOLUTION CONFORMATION AND BINDING-AFFINITY

The role of local steric influences on the solution conformation and t he biological activity of the II-type II blood group determinant 1 has been evaluated using structurally modified trisaccharides 2-4 and the ir corresponding C1-substituted C-glycosides 5-8 as conformational mod els. The preference of the C-glycosidic bond to adopt the gauche ''exo -anomeric'' conformation and the removal of destabilizing 1,3-diaxial- like interactions on the C-aglyconic bond have been used to create pre dictable conformational characteristics in C-trisaccharides 5-8. Vicin al coupling constants from H-1 NMR spectroscopy and 2D NOESY spectrosc opy demonstrate that structural modifications in the C-trisaccharides result in large changes in their conformational preferences. To test t he impact of solution conformation on receptor-ligand recognition, the affinities of compounds 1-8 toward the lectin I of Ulex europaeus (UE A-I) have been investigated using a quantitative binding assay. The bi nding affinities of the II-type II trisaccharide 1 and the correspondi ng carbon analog 5 are virtually identical. The activities of the stru cturally modified C-trisaccharides 6-8 decrease sharply relative to th e unmodified C-trisaccharide 5, correlating conformation to binding af finity. A parallel gradient in binding affinity is observed for the O- trisaccharides 1-4. The selectivity of UEA-I for epitopes 1-8 validate s the assumption that its receptor site largely defines a bound confor mation for the substrates, and establishes that the conformational beh avior of O-glycosides such as 1-4 is similar to that of C-glycosides s uch as 5-8.